In recent years, from the viewpoint of environmental protection, various electric devices have generally been controlled by inverter circuits for promoting energy saving and highly efficient operation. Particularly, in car industries, since a hybrid electric vehicle (HEV) which is driven by an electric motor and an engine is appeared on the market, car manufacturers have been promoting eco-friendly technology development focused on energy saving and high efficiency.
A motor for the HEV works with a range of voltage as high as several hundred volts. Considering above, manufacturers focus on a metallized film capacitor having preferable electric characteristics of a high withstand voltage and a low loss as being suitable for the motor. Besides, in response to demands of the market on maintenance-free components, the metallized film capacitor has been used because of its extremely long life time.
A metallized film capacitor for HEVs needs to have a high heat resistance and a high withstand voltage. To enhance these characteristics, many studies and suggestions have been made.
FIG. 9 is a perspective view of conventional metalized film capacitor 501 described in Patent Literature 1. Metalized film 21 includes dielectric film 22 of polypropylene and electrode film 23 that is metal-evaporated on a surface of film 22. Electrode film 23 extends in a longitudinal direction of film 21. Neither margin section 22A nor grid-like slit section 22B has electrode film 23 formed thereon. Fuse section 23B connects between segment sections 23A each of which serves as a function area constituting a unit capacitor. An evaporated electrode formed in the function area and an evaporated electrode formed in a electrode lead-out area are arranged in the longitudinal direction of film 21 and are separated by slit section 22C. Fuse section 23C connects between the evaporated electrodes in the two areas.
Metalized film 24 includes dielectric film 25 made of polypropylene and electrode film 26 that is metal-evaporated on a surface of dielectric film 25. Electrode film 26 extends toward in the longitudinal direction of dielectric film 21. Neither margin section 25A nor grid-like slit section 25B has electrode film 26 formed thereon. Fuse section 26B connects between segment sections 26A each of which serves as a function area constituting a unit capacitor. An evaporated electrode formed in the function area and an evaporated electrode formed in an electrode lead-out area are arranged in the longitudinal direction of film 21, and are separated by slit section 25C. Fuse section 26C connects between the evaporated electrodes in the two areas. Metalized film capacitor 501 has metal-sprayed electrodes 27 and 28 for leading out to external electrodes.
Conventional metalized film capacitor 501 is an assembly of the unit capacitors. Fuse sections 23B and 26B are provided between the unit capacitors, and fuse section 23C and 26C are provided between the function areas and the lead-out area of the capacitors. In response to anomalous condition, such as an excessively-large current flowing in the capacitor, fuse sections 23B, 23C, 26B, and 26C are cut to protect metalized film capacitor 501 from breakdown, thus reducing a decrease of a capacitance for maintaining the function of the capacitor. Even under a serious condition where fuse sections 23B and 26B cannot disconnect short-circuit currents at the breakdown, fuse sections 23C and 26C are cut the connection between electrodes 27 and 28 and the function area of the capacitor, thus protecting capacitor 501 from facing problems due to short-circuit.
In conventional metalized film capacitor 501, dielectric films 22 and 25 are made of polypropylene (PP) films. The upper withstanding temperature of PP film is low, about 110° C., which is much below a heatproof temperature of 150° C. required for capacitors used for vehicles.
In order to improve heatproof characteristics, dielectric films 22 and 25 may be made of dielectric film in which inorganic filler is added to a polymer material having a polar bond in its main chain. The polar bond includes an ester bond, an ether bond, an amide bond, and an imide bond. The materials that satisfy above are, for example, polyethylene naphthalate (PEN), polyphenylene sulfide (PPS), and polyethylene terephthalate (PET). However, the dielectric film made of the above materials has the following problem.
An electrode film manufactured by a metal-evaporation has a self-healing (SH) effect. Specifically, if an electrical breakdown occurs in an area with defective insulation, a metal-evaporated electrode disposed around the area having the breakdown evaporates and disperses by the energy occurred in short-circuit. The SH effect recovers an insulation property of the film. If the capacitor has a short-circuited part between the electrode films, the SH effect allows the capacitor to function properly. However, in the aforementioned dielectric film, which is made of a polymer material having a polar bond in its main chain and inorganic filler, the SH effect is low. That is, the film, such as polyethylene naphthalate (PEN), polyphenylene sulfide (PPS), and polyethylene terephthalate (PET), for capacitor 501 can hardly used instead of PP film.    Patent Literature 1: Japanese Patent Laid-Open Publication No. 8-250367